This invention generally relates to a large size magnetic coil for generating strong magnetic field, and more particularly to a combined superconducting coil having a high effective magnetic field, the coil and wires thereof being easily manufactured.
Superconducting magnets for generating strong magnetic field are essential for the plasma confining device of nuclear fusion and for the magnetohydrodynamic (MHD) power generator. At present, use is mostly made of a niobium-titanium alloy (NbTi) wire as the superconducting coil winding used for the magnets. More specifically, a so-called stabilized coil wire having a composite structure, wherein a plurality of fine or thin wires made of niobium-titanium alloy are embedded in a copper or aluminum matrix, has been practically used. This type of coil winding is referred to as "alloy type composite superconducting winding". This winding is fit for wire drawing process, so that long wires can be manufactured in large quantities by this process.
Ordinarily, a superconducting wire kept at a temperature lower than the critical temperature (or transition temperature) will lose its superconductivity under application of a magnetic field higher than a certain strength, and the maximum value of the magnetic field capable of maintaining the superconductivity of the wire is referred to as a critical magnetic field. In general, the critical magnetic field decreases according to the increase of the temperature of the wire and the increase of the current flowing therethrough. In the case of NbTi wire, if no current flow therethrough, this value is in a range of approximately from 10 to 12 T (wherein T means Tesla or Wb/m.sup.2) at the temperature of liquid helium; and when such a wire is actually wound into a large size coil for practical use and a rated current flows therethrough, it is considered that the critical magnetic field becomes approximately 8 T which corresponds to the maximum allowable value applicable to the winding.
On the other hand, when a magnet is composed of a combination of the superconducting coils, the effective working magnetic field, i.e. magnetic field effectively usable for the purpose of the superconducting magnet cannot be enhanced to the critical magnetic field. The reason follows; the maximum magnetic field actually applied to the winding itself is ordinarily greater than the effective working magnetic field because of the geometrical effect, and when this actually applied maximum magnetic field exceeds the critical magnetic field, the superconductivity of the magnet collapses, so that the magnet can no longer operate as a superconducting magnet.
The above described feature will be described in more detail with respect to a plasma confining magnet used in nuclear fusion reactors.
As for a magnet used for confining plasma, torus magnet is typical. In a practical nuclear fusion reactor, this magnet is ordinarily made of superconducting coils. In this case, several tens of superconducting coils of circular or D-shaped configuration dipped in liquid helium are disposed around the toroidal shape plasma vessel for generating toroidal magnetic field. In the practical nuclear fusion reactor, there exist further magnetic fields generated by other coils, such as of poloidal magnet creating a magnetic field in the poloidal direction (the direction perpendicular to and running around the toroidal magnetic field) in addition to the magnet for generating the toroidal magnetic field, thus exhibiting a complicated distribution of the magnetic field which is applied to the superconducting coil windings. The maximum magnetic field applied to each of the coil windings disposed in the toroidal form appears locally in a region of the coil facing the central axis of the torus and on the inner surface of the coil. This maximum magnetic field amounts to 1.5 to 3 times the effective working magnetic field of the toroidal magnet, i.e. the magnetic field in the central part of the cross section of the plasma vessel. Accordingly, if NbTi wire is used for the superconducting coil winding, the effective working magnetic field is held at a lower value in a range of from 3 to 4 T.
There has been a strong demand for improving the effect of the magnetic field by generating higher effective working magnetic field not only in the nuclear fusion magnet but also in other superconducting magnets used for various purposes, and the development of superconducting wires for higher magnetic field, which have higher critical magnetic field values than the NbTi, has been pursued energetically. At present, winding wires having composite structures using intermetallic compounds, such as Nb.sub.3 Sn, V.sub.3 Ga, or the like (hereinafter referred to as "compound type materials"), are manufactured as the superconducting wires for higher magnetic field. While some of these wires have critical magnetic field values exceeding 20 T, the compound type wire materials are hard and brittle, so that these materials cannot be drawn into wires. Therefore, it is said that the compound type composite superconducting wire of long size can be hardly manufactured economically as in the case of the alloy type superconducting wire, and it is considered that large superconducting coil made of compound type composite superconducting wire is hardly developed economically in near future.